The reflective liquid crystal display (LCD) 100 for projection application as depicted in FIG. 1 has been used for front projectors and rear projection TVs. The whole system includes a polarizing beam splitter (PBS) 110 and a reflective liquid crystal (LC) panel 120. Several LC modes have been used, such as (1) vertical alignment (VA) mode disclosed in M. F. Schiekel and K. Fahrenschon, “Deformation of Nematic Liquid Crystals with Vertical Orientation in Electrical Fields”, Appl. Phys. Lett., Vol. 19, p. 391, (1971), (2) 45° hybrid field effect (HFE) mode discussed in J. Grinberg, et al, “A New Real-Time Noncoherent to Coherent Image Converter: the Hybrid Field Effect Liquid Crystal Light Valve”, Opt. Eng., Vol. 14, p. 217, (1975), (3) 63.6° twisted nematic-electronically controlled birefringence (TN-ECB) mode disclosed in T. Sonehara, “Photo-Addressed Liquid Crystal SLM with Twisted Nematic ECB (TN-ECB) Mode”, Jpn. J. Appl. Phys., Vol. 29, L1231, (1990), and (4) mixed twisted-nematic (MTN) mode disclosed in S. T. Wu and C. S. Wu, “Mixed-Mode Twisted Nematic Liquid Crystal Cells for Reflective Displays”, Appl. Phys. Lett., Vol. 68, p. 1455, (1996).
In a high resolution projection display, the pixel size is comparable to the cell gap. The fringing field between adjacent pixels could reorient the LC directors and then degrade the image contrast ratio and reduce display brightness. See, for example, K.-H. Fan Chiang, S. T. Wu and S. H. Chen, “Fringing Field Effect of the Liquid-Crystal-on-Silicon Devices”, Jpn. J. Appl. Phys. Vol. 41, p. 4577, (2002). Therefore, to decrease the fringing field effect, low driving voltage is preferred for achieving high resolution, high contrast ratio and high brightness projection display. Although the MTN mode can reach a relatively low voltage compare to other modes, its fringing field is still prominent.
In order to get a low driving voltage, U.S. Pat. No. 5,490,003 issued to Van Sprang on Feb. 6, 1996 reveals a reflective LCD using positive dielectric anisotropic LC material with the entrance polarization direction set at the bisector of twist angle, and U.S. Pat. No. 5,936,697 issued to Yang on Aug. 10, 1999 proposed the self-compensated TN concept using negative dielectric anisotropic LC materials. Both patents set the entrance polarization direction at the bisector of twist angle in order to achieve a self-compensation effect. In addition, the self-compensated TN mode and bisector effect in transmissive and reflective TN cell were published in K. H. Yang, “A Self-Compensated Twisted-Nematic Liquid Crystal Mode for Reflective Light Valves”, Euro display'96, p. 449 (1996) and S. T. Wu and C. S. Wu, “Bisector Effect on the Twisted-Nematic Cells”, Jpn. J. Appl. Phys., Vol. 37, L1497, (1998), respectively. These two publications refer to the bisector effect of a TN cell.
The present invention advances the art by providing a method and a device that positions the entrance polarization direction of PBS to form an angle β with respect to the rubbing direction of the top substrate, where β angle deviates from the bisector of twist angle which is used in the prior art. Using the entrance polarization angle of PBS determined by the present invention, the contrast ratio increases dramatically and the driving voltage decreases significantly. Decreasing the driving voltage also minimizes the fringing field effect of the reflective liquid crystal cell. By taking the boundary layer residual phase retardation into consideration, the optimal entrance polarization angle is determined to be approximately 1-3° higher than the prior art bisector effect to achieve an approximately perfect dark state at the decreased driving voltage.